SWAPON(8) Linux Programmer's Manual SWAPON(8)
swapon, swapoff - enable/disable devices and files for paging and swapping
swapon -s [-h] [-V]
swapon [-d] [-f] [-p priority] [-v] specialfile...
swapoff [-v] specialfile...
swapon -a [-e] [-f] [-v]
swapoff -a [-v]
swapon is used to specify devices on which paging and swapping are to take place.
The device or file used is given by the specialfile parameter. It may be of the form -L
label or -U uuid to indicate a device by label or uuid.
Calls to swapon normally occur in the system boot scripts making all swap devices avail-
able, so that the paging and swapping activity is interleaved across several devices and
swapoff disables swapping on the specified devices and files. When the -a flag is given,
swapping is disabled on all known swap devices and files (as found in /proc/swaps or
All devices marked as ``swap'' in /etc/fstab are made available, except for those
with the ``noauto'' option. Devices that are already being used as swap are
Discard freed swap pages before they are reused, if the swap device supports the
discard or trim operation. This may improve performance on some Solid State
Devices, but often it does not. The /etc/fstab mount option discard may be also
used to enable discard flag.
Silently skip devices that do not exist. The /etc/fstab mount option nofail may be
also used to skip non-existing device.
Reinitialize (exec /sbin/mkswap) the swap space if its page size does not match
that of the the current running kernel. mkswap(2) initializes the whole device and
does not check for bad blocks.
Use the partition that has the specified label. (For this, access to /proc/parti-
tions is needed.)
-p, --priority priority
Specify the priority of the swap device. priority is a value between 0 and 32767.
Higher numbers indicate higher priority. See swapon(2) for a full description of
swap priorities. Add pri=value to the option field of /etc/fstab for use with
Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not avail-
able before Linux 2.1.25.
Use the partition that has the specified uuid.
You should not use swapon on a file with holes. Swap over NFS may not work.
swapon automatically detects and rewrites swap space signature with old software suspend
data (e.g S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't do it, then we get
data corruption the next time an attempt at unsuspending is made.
swapon may not work correctly when using a swap file with some versions of btrfs. This is
due to the swap file implementation in the kernel expecting to be able to write to the
file directly, without the assistance of the file system. Since btrfs is a copy-on-write
file system, the file location may not be static and corruption can result. Btrfs actively
disallows the use of files on its file systems by refusing to map the file. This can be
seen in the system log as "swapon: swapfile has holes." One possible workaround is to map
the file to a loopback device. This will allow the file system to determine the mapping
properly but may come with a performance impact.
swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8)
/dev/sd?? standard paging devices
/etc/fstab ascii filesystem description table
The swapon command appeared in 4.0BSD.
The swapon command is part of the util-linux package and is available from ftp://ftp.ker-
Linux 1.x 25 September 1995 SWAPON(8)